Shaping a Circular Future with Bio-derived Resins in Additive Manufacturing.

The integration of circular principles into chemical manufacturing is poised to significantly transform the production of plastics. This shift will have impacts across the value chain, including raw material sourcing, recyclability, and cost. In recent years, interest in photopolymer-based additive manufacturing (AM) has grown, driven by the increasing demand for multifunctional 3D printing. Photopolymer printing is widely applied in academia and industry as it provides high resolution and rapid print speeds and allows for the construction of plastic products with complex geometries. Therefore, in addition to the benefits of using energy from light, rather than higher energy thermal curing, to create the printed materials, AM offers the advantages of waste reduction through dematerialization. Among the photopolymer-based AM techniques, digital light processing (DLP) is commonly applied due to its high accuracy and resolution and low capital cost for equipment. While advances in equipment are making production-scale photopolymer printing a reality, resin design is largely embedded in the chemistry of the past. However, the nascent state of this industry presents an opportunity to embed sustainability within its material set as it grows, preventing many of the environmental and human health impacts that are linked with the current use of petrochemically derived plastics. While furthering functionality or performance of the materials produced remains an ongoing target for the field, to enhance its sustainability credentials research is focused on (i) switching from petrochemical to biomass-derived monomer feedstocks and (ii) enhancing material circularity such that resins can be printed, depolymerized, and re-printed in a closed-loop, circular manner. Achieving these milestones requires consideration of resin feedstock sourcing and the design of monomers with dynamic bonds to enable recyclability and reprocessability. To expand the portfolio of biomass-derived photopolymer resins, we, along with others, have explored the use of bio-derived and bio-derivable (i.e., those chemical feedstocks that have the potential to be bio-derived but at present are not) monomers that are easily derivatized and compatible with DLP systems. Typical approaches leverage the reactive functional groups in bio-derived monomer sources to create acrylates or epoxides that enable rapid crosslinking, to achieve high-quality 3D-printed polymers. Our approach has focused on leveraging double bonds that naturally occur in biomass-derived chemicals to reduce the number of reaction steps by promoting polymer formation through direct reaction of that double bond via thiyl radical addition chemistry. This step-growth addition method has enabled us to create materials that can be fully degraded to small molecules, but it has also provided opportunities to leverage double-bond stereochemistry to achieve photosets with tunable mechanical properties. Monomer bio-sourcing, however, only addresses half of the problemthe absence of dynamic chemistry inherently limits the recyclability of the resulting materials, thus leading to what is printed becoming waste. To achieve circular resins, leveraging dynamic covalent chemistry has been key to enabling the fabrication of materials that can be readily recycled and reprocessed. While many approaches require monomers to be added to depolymerized resins in an "open-loop" manner, our approach focuses on disulfide chemistry that can be fully returned to its initial state and then re-printed in a "closed-loop" manner. Ultimately, applying bio-derived monomers, circular resin systems, and eco-friendly manufacturing methods is essential to building a truly sustainable manufacturing ecosystem capable of scaling from the laboratory to production. This Account focuses on the innovations that enable sustainable, high-performance additive manufacturing.